In many semiconductor processing environments, it is necessary to sever, or "open circuit", selected conductive lines on a semiconductor chip or substrate. The analysis of a failed semiconductor chip, for example, often requires that selected conductors be open circuited so that suspected failing structures are electrically isolated from surrounding circuits. This isolation permits examination of the suspected defective part devoid of electrical interference from the surrounding electronics.
Another example of an application utilizing the open circuiting of conductive lines is that of isolating known bad circuits and/or components on an otherwise operable semiconductor chip. Such utilization can result in a substantial yield improvement for devices such as memories wherein inoperable circuits can be physically and logically isolated to provide a good device.
As advanced semiconductor manufacturing techniques permit the construction of smaller, more densely packed structures, the isolation and open circuiting of selected conductors becomes increasingly difficult. Very large scale integrated circuits (VLSI), for example, may include conductors sized on the order of 0.7 microns, and these conductors can be spaced at intervals of 0.7 microns. Thus, very delicate processes are necessary to isolate and open circuit a single one of these conductors.
Currently, several processes are known for open circuiting conductors on semiconductor chips, including: photolithographic masking with chemical etching, high-power laser cutting, and mechanical polishing or scribing. Each of these processes, however, is known to exhibit substantial disadvantages.
The use of photolithographic masks and chemical etching of the conductor is a slow, complex process requiring multiple processing steps. Further, it is difficult to isolate only the desired conductor, and difficult to limit the extent of the etching.
The use of high power lasers can cause damage to both underlying and surrounding structures. More particularly, the intensity of the power required to vaporize the conductive lines makes it extremely difficult to terminate the laser after vaporizing a line before substantial damage is done to the underlying substrate. Further, the conductive material vaporized by the laser can contaminate other regions on the substrate.
Mechanical scribing and polishing is difficult to control to the necessary tolerances, and can cause damage to surrounding circuits.
U.S. Pat. No. 3,597,579 to Lumley shows a method of laser trimming a capacitor while maintaining an electrical potential between the capacitor plates so as to inhibit the formation of electrical shorts.
U.S. Pat. No. 4,582,976 to Merrick shows a method of laser trimming a resistor in a temperature compensating resistive circuit.
U.S. Pat. No. 4,358,659 to Spohnheimer shows a method of maintaining a laser beam in focus on the surface of a semiconductor circuit.
None of these patents show or suggest a desirable means for open circuiting a selected conductor on a semiconductor substrate.